I am trying to use xdmcp on 8.04. I use firehol and have opened the xdmcp service. I was able to create a session with the remote machine but only got a black screen with a "X". I then opened tcp/udp ports 6000-6063 for x11 and got a tan background with a white box in the top left of the screen. It seems as if there are more ports that need to be opened up.
XDMCP works wonderfully when I drop the firewall.